This invention is concerned with accurately counting the passage of workpieces on a conveyor system or production line path, and more particularly, it relates to an improved method of sensing interruptions in the path of a light beam or other sensor at a preset point along a conveyor and, with a computer, applying a mathematical relationship to the measured durations of these interruptions and the time periods separating them, to determine which interruptions should be counted as representing the passage of conveyed workpieces and avoiding accidental counting of other interruptions.
In the printing industry, problems occur when attempting to optically count the high speed passage of single sheets of paper along a conveyor, especially where the conveyor speed is variable, gaps occur between sheets and sheets may be missing at various locations along the conveyor. Although other types of sensors can be used, typically an optical sensor has been used to detect breaks in the path of a beam of light, the breaks being caused by the passage of intervening sheets of paper.
On a printing press or other paper handling production line, sheets of paper moving along a conveyor are typically pulled by gripper clips attached to a gripper bar. These gripper bar and clip devices precede each sheet of paper along the path of the conveyor. Consequently, when sheets of paper are sequentially pulled through the path of a light beam, the gripper bar and possibly the clip device will also cause an interruption to the path of the light beam. A common practice in the industry has been to simply count up the number of light beam interruptions and divide this figure by two, calling this the sheet count. This method is based on the assumption that for each light beam interruption caused by a sheet of paper, another interruption will be caused by the passage of the attached gripper bar. This assumption is faulty for several reasons. First, when a sheet is missing at a particular location on the conveyor, its gripper bar and clip still cause a light beam interruption, thus causing errors in the count. Secondly, a particular positional alignment of the gripper clip may periodically cause a long duration interruption to the light beam as the gripper bar and clip and sheet of paper all pass through the path of the light beam together, causing only one interruption and again introducing an error. This single long interruption is caused first by the gripper bar, then by the gripper clip, then by the sheet of paper, with no separation between these interruptions.
Another practice in the industry has been to measure the time durations of light beam interruptions and then to try to discern whether each interruption is caused by the passage of a sheet of paper. The major limitation to this time measurement method is its inability to account for changes in conveyor speed.
It is an object of this invention to overcome the above problems and to accurately count sheets of paper passing along a variable speed conveyor such that these sheets of paper can be reliably distinguished from gripper bars and clips; a broader object is to accurately count conveyed workpieces in general, where conveyor components could be confused with workpieces.